1. Field of the Invention
The present invention relates to a substrate with transparent conductive layer(s), more particularly a substrate for a photovoltaic element with transparent conductive layer(s) capable of improving the yield in the production process and reliability such as weatherability or durability while improving the efficiency of photoelectric conversion by use of light confinement effect, and a photovoltaic element using the same. The photovoltaic element is one used for solar cells, photodiodes, electrophotographic photosensitive materials or the like.
2. Related Background Art
A photovoltaic element for converting light into electric energy has been widely applied to a small power source for public appliances, such as electronic calculators or wrist watches, as a solar cell and attracts attention as a technique practicable for future substitutes to so-called fossil fuels such as petroleum and coal. It is also used for facsimile, scanner or the like as a sensor and also for an electrophotographic photosensitive drum in copiers or the like. The photovoltaic element utilizes the photoelectromotive force of a semiconductor p-n junction or photoelectric conversion of a semiconductor, in which light is absorbed into a semiconductor such as silicon to generate photocarriers of electrons and electron holes to drift and remove them by the internal electric field of a p-n junction remove.
Photovoltaic elements that have been thus far used most generally are those constructed with single-crystalline silicon. Such a photovoltaic element can be manufactured by a process similar to a normal semiconductor process. Specifically, a single crystal of silicon subjected to valence electron control into n-type or p-type is prepared by a crystal growth method such as a CZ method, and the single crystal is sliced to make silicon wafers about 300 μm in thickness. Furthermore, a p-n junction is made up by forming a different-type conductive layer on the surface of a wafer under application of a valence electron control agent by appropriate means such as diffusion so as to form a layer of a conductivity type opposite to that of the above wafer.
The production cost for a photovoltaic element using such single-crystalline silicon, however, is increased. Because of a high cost in making silicon wafers and a high manufacturing cost due to the use of a semiconductor process, the photovoltaic element becomes rather expensive for the amount of unit electricity generated compared to an existing power generating method. It is considered difficult to lower the production cost to a level for power generation.
Thus, in advancement into a practical use for the power source of a photovoltaic element, cost reduction and area increasing are recognized to be important technical problems, and materials that are low in cost or high in conversion efficiency have been searched.
As a material for such photovoltaic elements, a tetrahedral amorphous semiconductor such as amorphous silicon, amorphous silicon germanium, and amorphous silicon carbide, or a polycrystalline semiconductor, or a semiconductor of the groups II–VI compounds such as CdS or Cu2S or a semiconductor of the groups III–V compounds such as GaAs or GaAlAs can be referred. Above all, a thin-film photovoltaic element using an amorphous semiconductor or a polycrystalline semiconductor as a photoelectromotive force generating layer is advantageous in that it enables preparation of a larger area film than a photovoltaic element using single-crystalline silicon. A film can be thin, and a film layer can be deposited onto any supporting substrate material and regarded as promising.
In the above photovoltaic element, however, photoelectric conversion efficiency comparable to that of a photovoltaic element using single-crystalline silicon has not yet been obtained. Improving the photoelectric conversion efficiency and improving the reliability have been a problem to be studied to make this element practicable as a power element.
Thus, various methods have been examined as means for improving the photoelectric conversion efficiency of a film photovoltaic element.
One of the important problems in improving the photoelectric conversion efficiency of a film photovoltaic element is to increase the light absorption in a film semiconductor layer and to increase the short-circuit current (Jsc). This is because making the semiconductor layer thinner for cost lowering leads to a smaller optical absorption than that of a bulk semiconductor. Several techniques have been examined for increasing the light absorption in a film semiconductor layer.
As one of them, a technique for growing a reflective layer, also serving as a back face electrode, made of a metal film with a high reflectivity such as Ag, Al, Cu or Au, is known. This technique intends to reflect the light transmitted through the semiconductor layer generating carriers reflected by the reflective layer so as to be absorbed by the semiconductor layer again, thereby increasing the light absorption in a film semiconductor layer, and increasing the output current to improve the photoelectric conversion efficiency.
On the other hand, a method for improving the substrate surface property by laying a transparent conductive film between the back face electrode and the semiconductor layer is disclosed in Japanese Patent Publication No. 59-43101 and Japanese Patent Publication No. 60-41878. As the effect of laying a transparent conductive film between the back face electrode and the semiconductor layer, improving the evenness property of a back face electrode, improving the close adhesion of a semiconductor layer, or preventing the alloying between the metal of a back face electrode and a semiconductor layer or the like is referred to in these Patent Publications.
Besides, Japanese Patent Application Laid-Open No. 60-84888 discloses a technique for reducing the current flowing through the defect region of a semiconductor layer by laying a transparent conductive film between the back face electrode and the semiconductor layer as the barrier layer.
Besides, it is reported at p. 644 of Appl. Phys. Lett., 43 (1983) by Y. Hamakawa et al. that the spectral sensitivity of a long-wavelength region is increased by laying a transparent conductive film made of TiO2 between the Ag back face electrode and the amorphous silicon semiconductor layer.
Besides, disclosed at p. 1423 of Proc. 16th IEEE Photovoltaic Specialist Conf. (1982) by T. Tiedje et al and p. 1425 of Proc. 16th IEEE Photovoltaic Specialist Conf. (1982) by H. Deckman et al. is a technique for allowing the long-wavelength light not absorbed into a semiconductor to be scattered on a back face electrode, the which is formed into an uneven shape (texture structure) having a size of about the light wavelength, thereby increasing the optical path length in the semiconductor layer and improving the long-wavelength sensitivity of a photovoltaic element to increase the short-circuit current and improve the photoelectric conversion efficiency.
On summarizing these techniques, a constitution is considered to be the most suitable for a photovoltaic element wherein a metal layer having an uneven shape and about a light wavelength size for scattering light and a high reflectivity is formed as a back face reflective layer serving simultaneously as a back face electrode and a transparent conductive film is interposed between the back face reflective layer and a semiconductor layer.
When an attempt is made to actually manufacture a photovoltaic element by adopting a back face electrode of such a constitution, several problems appeared from the viewpoint of workability or durability.
Thus far, a typical uneven form referred to as a so-called texture structure, e.g., the one having a pyramid-shaped unevenness as illustrated in the above literature of T. Tiedje et al., has been considered to be excellent in confinement effect. However, there is the need for further examining which shape is the most suitable for improving the efficiency and yield and for improving the operativity in the subsequent steps.
First of all, in a semiconductor layer of a pyramid-shaped unevenness having steep vertexes or valleys formed on the surface, a stress is generated locally at the vertex of a pyramid shape and a defect portion is likely to be created in the semiconductor layer. Besides, when an electromotive force is generated, the leakage current of a photovoltaic element through the defect portion or the like of the semiconductor layer may increase by the concentration of the electric field at the vertex of the pyramid shape, thereby lowering the manufacturing yield of the photovoltaic element.
Especially, in the case of depositing the semiconductor layer at high speed, e.g., at a deposition speed of 10 Å/s or more, the adherence of a film is liable to become uniform, so that a fault in the deposition of a film at the valley between pyramid shapes or a peeling of the film from the vertexs may be observed.
Besides, in a semiconductor layer formed on the uneven surface of pyramid shapes having uniformly steep vertexes or valleys, the electric field became more intense at the pyramid vertexes than in the semiconductor layer formed on a flat surface, so that there were cases where a nonuniform electric field led to a decrease in the open voltage (Voc) and the fill factor (FF) of a photovoltaic element as compared with a photovoltaic element formed on a flat supporting-substrate surface.
Furthermore, an increase in the optical degradation (worsening of element characteristics due to long-time light illumination) and the vibrational degradation (worsening of element characteristics due to long-time application of vibration) of a photovoltaic element may be observed. Namely, the optical degradation of a photovoltaic element is considered to be generated by breaking weak bonds by light energy to form recombination centers of photoexcited carriers, thereby worsening the element characteristics. Also, the vibrational degradation of a photovoltaic element is considered to be generated by breaking weak bonds by vibration energy to form recombination centers of photoexcited carrier, thereby lowering the element characteristics. These weak bonds are considered to be localized in a region where a stress is generated.
When a semiconductor layer is deposited at a high speed, pin holes are especially likely to be generated, whereas a short circuit may occur in the semiconductor layer when the transparent conductive layer with a high conductivity is used.
Besides, in the case of using Ag or Cu as the back face metal reflective layer, a migration of Ag or Cu was found to occur at high humidity and under application of a positive vias voltage to the back face metal reflective layer, so that a conduction took place between the reflective layer and the electrode on the light incident side and the photovoltaic element shunted (was short-circuited).
On the other hand, when a transparent conductive layer is formed in a flat shape, there were problems of an insufficient light absorption in the semiconductor layer because of a small scattering of light on the back face, and at peeling that might occur between the supporting substrate and the back face electrode in the working step of a photovoltaic element because some combination of materials for the supporting substrate and the back face electrode led to an insufficient adhesion between the supporting substrate and the back face electrode.
In the step of removing the electrically short circuit in a faulty portion as the subsequent step, a reaction may proceed excessively at the steep vertex of a pyramid shape on a surface having a pointed unevenness, thereby damaging a place where there is no defect. Consequently, in such a substrate, the setting range of conditions in the step of removing the electrically short circuit in defect portion is compelled to be narrowed. This requires a strict control over the removing step, thereby lowering productivity.
In the step of removing the short circuit in a faulty portion as the subsequent step, a lower corrosion resistance of transparent conductive film may cause the transparent conductive layer to be etched through pin holes, or lead to film peeling or induction of a short circuit by contraries, thereby lowering reliability.
These problems were conspicuous especially in the case of adopting a cost-saving manufacturing step suitable for a practical implementation by using an inexpensive supporting substrate such as resin film or stainless film or by increasing the production speed as a result of an increase in the formation speed of a semiconductor layer, and are a factor in lowering the yield of manufacturing a photovoltaic element.